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Two former Civil Aviation crash investigators have told the inquest into the Fox Glacier skydiving air disaster they do not believe excess weight contributed to the crash that killed nine people.

Four overseas tourists and five Skydive NZ staff were killed instantly when the Fletcher FU turbo-prop plane plunged into a paddock at the end of the Fox Glacier airstrip and exploded in a fireball on September 4, 2010, after the pilot lost control on take-off.

An earlier Transport Accident Investigation Commission report concluded that excess weight and a load shift caused the fatal flight, as the unfastened passengers slipped to the back of the plane, causing it to lose balance.

However, former Civil Aviation investigator Joseph Daly told the inquest at Greymouth yesterday that the plane, in its previous life as a crop duster, had frequently carried much more than it did on the day of the fatal flight, without incident.

The plane was licensed to carry 2204kg as a skydive plane but had been licensed to carry 2463kg as a crop duster, and was entitled under the agricultural flight regulations to carry 30 per cent more than that and sometimes did.

"I believe that weight was not a causative issue," Mr Daly said.

Another former Civil Aviation safety inspector, Mark Houston, also disregarded weight as a factor.

He also doubted that load shift had been a factor, and instead believed that a gear failure had been the cause.

He said it was possible that the rearward jumpmaster in the plane might have pushed back through the thin metal bulkhead and his parachute somehow interfered with the controls, but he agreed that was speculation.

Mr Houston accepted under cross-examination that it was only three weeks ago that he was asked, by former Skydive NZ director John Kerr, to give evidence at the inquest.

However, he said the weight and balance hypothesis was also just speculation and in his view was "very unlikely" to be the cause of the accident.

Other possibilities were cable failure or the control column being inadvertently locked in place.

He would have liked to have examined the control column, but it had been buried at the crash site.

Mr Houston said that in a very recent incident the control column had locked in flight and the pilot was fortunate on that occasion that it was a dual control plane and he had another column to fly the plane with.